3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as enhanced or evolved Node Bs (eNBs)) to mobile stations (e.g., User Equipment devices (UEs)) are sent using Orthogonal Frequency Division Multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency. The basic unit of transmission in LTE is the Resource Block (RB), which in its most common configuration consists of 12 subcarriers in frequency and 7 OFDM symbols in time (one slot). A unit of one subcarrier and 1 OFDM symbol is referred to as a Resource Element (RE), as shown in FIG. 1. Thus, an RB consists of 84 REs.
An LTE radio subframe is composed of two slots in time and multiple RBs in frequency with the number of RBs determining the bandwidth of the system, as illustrated in FIG. 2. Furthermore, the two RBs in a subframe that are adjacent in time are denoted as an RB pair. Currently, LTE supports standard bandwidth sizes of 6, 15, 25, 50, 75, and 100 RB pairs. In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
The signal transmitted by the eNB in a downlink (the link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas, and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE relies on Reference Symbols (RS) that are transmitted on the downlink. In addition, reference signals can be used to measure the channel between the transmitter and the receiver antenna. Therefore, Antenna Ports (AP) are introduced in the LTE specifications. Each RS is associated with an AP, and when the UE is measuring the channel using the RS, it is said that the UE is measuring the channel from the stated AP (to the receiver antenna). It should be noted that it is up to transmitter implementation to determine how to transmit the RS in case there are multiple physical antennas at the transmitter side used to transmit the RS for a single port. The mapping of a RS to multiple physical antennas is called antenna virtualization and this operation is transparent to the UE since the UE can only measure the channel on the given RS, i.e. the AP.
The RSs and their position in the time-frequency grid are known to the UE and hence can be used to synchronize to the downlink signal and determine channel estimates by measuring the effect of the radio channel on these symbols. In Release 11 and prior releases of LTE, there are multiple types of RSs. The Common Reference Signals (CRSs), corresponding to AP 0-3, are used for channel estimation during demodulation of control and data messages in addition to synchronization. The CRSs are present in every subframe. The Channel State Information Reference Signals (CSI-RSs, AP 15-22) are also used for channel state feedback related to the use of transmission modes that enable UE-specific antenna precoding. These transmission modes use the UE-specific Demodulation Reference Symbols (DM-RSs, AP 7-14) at the time of transmission with the precoding at the eNB performed based on the feedback received from and measured by the UE on the CSI-RSs. The Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS) are used for cell search and coarse time and frequency synchronization. These signals are strictly not reference signals but synchronization signals and hence do not correspond to any numbered AP in the LTE specifications. All of these reference signals are shown in FIG. 3 over two subframes of duration 1 ms each.
The CSI-RSs are modulated using a sequence that depends on a configurable cell Identifier (ID) that can be different from the cell ID being used in the cell. The CSI-RS utilizes an orthogonal cover code of length two to overlay two APs on two consecutive REs. As shown in FIG. 3, many different CSI-RS patterns are available. For the case of two CSI-RS APs, there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS APs, respectively. For Time Division Duplexing (TDD), some additional CSI-RS patterns are available.
The CSI-RS can be configured for a UE as Non-Zero-Power (NZP) and Zero-Power (ZP) instances. The NZP CSI-RS configuration indicates the REs where the cell being measured transmits CSI-RS and the ZP CSI-RS configuration indicates the REs where no information is transmitted by the cell being measured. The ZP CSI-RS REs are typically configured so that they overlap with transmissions from other cells which allows the UE to make interference measurements or Reference Signal Received Power (RSRP) measurements on the CSI-RS of other cells. Knowledge of the ZP CSI-RS configurations also allows the UE to not use these REs, i.e., rate-match around these REs when receiving the Physical Downlink Shared Channel (PDSCH).
The PSS and SSS define the cell ID of the cell. The SSS can take 168 different values representing different cell ID groups. The PSS can take three different values that determine the cell ID within a group. Thus, there are a total of 504 cell IDs. FIG. 4 illustrates reference signals in Frequency Division Duplexing (FDD) and TDD carriers.
Dense deployments of small cells are attractive to increase system capacity. However, dense deployments typically have fewer UEs connected to each cell and lower resource utilization with higher rates provided when the cells are used. Reference signal structures that are developed for regular deployments with existing systems such as 3GPP LTE may have too high of a density so that there is a substantial amount of unnecessary interference created when deployments become dense. Reference signals may be transmitted even when there is no data being sent to the UEs.
In order to tackle this problem of unnecessary interference, solutions to turn small cells off when they are not being used are being introduced in 3GPP LTE Release 12. However, to ensure that cells can be ready to deliver data to and receive data from UEs with minimal delay, it is necessary for UEs to make some essential measurements on cells even when they are off. In order to facilitate this, a set of reference signals that are sent with much lower density in time have been introduced. Such signals are referred to as discovery signals and procedures associated with them are referred to as discovery procedures.
More specifically, in LTE Release 12 small cell on/off where the eNB can be off for long periods of time, a discovery signal can be configured in order to assist the UE with the measurements. The discovery signal supports the properties required for enabling Radio Resource Management (RRM) measurements (e.g., received power and quality measurements (referred to as RSRP and Reference Signal Received Quality (RSRQ) measurements in LTE)) and time/frequency synchronization. The discovery signals are sent in a Discovery Reference Signal (DRS) occasion that can have a duration from 1 to 5 subframes for FDD and 2 to 5 subframes for TDD. The DRS occasions can occur once every 40, 80, or 160 ms. The UE is configured with a Discovery Measurement Timing Configuration (DMTC) for each carrier frequency on which RRM measurements for cells needs to be performed. The DMTC duration is 6 ms and the timing of the DMTC is signaled to the UE in reference to the current serving cell.
Within one cell, there may be multiple transmission points from which the downlink signal can transmitted. The transmission points may be geographically separated within the cell and/or correspond to antennas with significantly different coverage areas. Examples of this are a distributed antenna system that transmits signals that all belong to the same cell (i.e., the same cell ID), where multiple radio remote heads are physically dislocated within the cell. The term transmission point may also refer to a sector of a site where the different sectors of the same site then constitute different transmission points. The discovery signal is capable of identifying individual transmission points and enabling RRM measurements for them via the use of different CSI-RS configurations at different transmission points.
In LTE, the CSI-RSs can be assigned to different transmission points within a cell to identify them. The CSI-RSs are part of the discovery signal being introduced in Release 12 and simple RSRP measurements on the CSI-RSs are being defined. The CSI-RSs have a high degree of configurability and are designed to be used for CSI measurements by the UE. However, due to the high degree of configurability, the UE needs to be provided assistance information by the network about the precise configuration that the UE should use for measurements.
The NZP and ZP CSI-RS configurations that are part of the discovery signal are configured semi-statically via higher layer Radio Resource Control (RRC) signaling as part of the discovery signal configuration. The discovery signal configuration also indicates the periodicity of the discovery occasions which may occur once every 40, 80, or 160 ms. These discovery occasions may last up to 5 subframes in duration on each occasion. The UE may also be configured with NZP and ZP CSI-RS configurations for CSI feedback independently from the discovery signal configuration. These CSI-RSs can occur as often as once every 5 ms.
If the CSI-RSs are configured separately for CSI feedback and for discovery signal based RRM measurements and they provide different information on which REs are configured to be ZP and NZP, it may create a problem for the UE. In particular, the UE does not know how to process the subframe when receiving PDSCH and use the CSI-RS for feedback or measurements.
Existing solutions as discussed in LTE Release 12 rely on the eNB using different CSI-RS configurations for the discovery signal subframes and for other subframes where CSI-RSs are configured for CSI feedback. If subframes with CSI feedback do overlap with the discovery signal subframes, existing solutions rely on the CSI-RS configurations being the same between them so that there is no conflict. This creates restrictions for how the CSI-RSs can be configured for regular CSI feedback, which is undesirable.